I am using the gLTF addon, how do I make my node group glTF Metallic Roughness exactly the same as my Diffuse BSDF
They're different shading models; it may not be possible to exactly match one with the other and vice versa. If you're creating a model for export into some 3D engine, you should try importing both and verify that it looks correct there, as ultimately the engine will determine how it looks.

In the future the glTF Blender Exporter will work with the Principled BSDF node, and then you won't need a special addon glTF PBR node like this.